Over 170 million people world-wide are infected with hepatitis C virus (HCV), a major cause of liver cirrhosis and hepatocellular carcinoma. Despite the global prevalence of HCV, little is known about how this virus interacts with the host and evades the immune system to establish persistent infection. HCV circulates in infected individuals as a heterogeneous population based on buoyant density and this heterogeneity is due in part to virus association with lipoproteins - including very low density (VLDL), low density (LDL) and high density (HDL) lipoproteins. HCV association with lipoproteins initiates during virus particle assembly, and release of progeny HCV is closely linked to the VLDL secretory pathway. Entry into susceptible cells is also facilitated by interaction with lipoproteins, specifically HDL, which promotes rapid kinetics of entry thought to facilitate escape from neutralizing antibodies. Together, these data support the interaction between HCV and lipoproteins and underscore their role in HCV pathogenesis, however, the precise nature of how these lipoproteins interact with HCV is unknown and the full consequences of their interaction not fully explored. Our overall hypothesis is that HCV virion association with, or incorporation of, host proteins and lipids reduces virus susceptibility to neutralization. Previous studies indicate that lipoprotein interaction is mediated by the HCV surface glycoproteins, E1 and E2. In Specific Aim 1, we will define the specific molecular requirements for lipoprotein (specifically HDL)-HCV interaction using both biochemical assays to dissect the HDL complex and a glycoprotein mutagenesis strategy to identify critical viral residues for binding. Independent of viral infection, lipoproteins have been shown to have immunoregulatory activities, and HDL has specifically been shown to modulate complement activation. Thus, in Specific Aim 2 we will investigate the impact of HCV association with HDL on virus susceptibility to complement-mediated virolysis. Our final Specific Aim is to perform complete proteomic and lipidomic analyses on purified HCV. These data will not only further elucidate the lipoprotein-virus interaction, but will also enable us to catalog the cellular components of the virus particle. Such analyses will provide valuable insight into the composition of the HCV particle and its life cycle.